• Nuclear Engineering and Technology
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• v.53 no.2
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• pp.666-676
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• 2021

This paper presents numerical estimation of burst pressures of axial cracked U-bend tubes, considering the U-bending process analysis. The validity of the FE simulations is confirmed by comparing with published experimental data. From parametric analyses, it is shown that existing EPRI burst pressure estimation equations for straight tubes can be conservatively used to estimate burst pressures of the U-bend tubes. This is due to the increase in yield strength during the U-bending process. The degree of conservatism would decrease with increasing the bend radius and with increasing the crack depth.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.9
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• pp.1207-1217
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• 1997

The present work experimentally investigated the effects of mass flux, heat flux, inlet quality on the heat transfer performance inside the U-bend of smooth and microfin tube using R-22 and R-407C refrigerants. The parameters were 200 and 400 kg/m$^{2}$ s for mass flux, 6 and 12 kw/m$^{2}$ for heat flux, 0.1 and 0.2 for inlet quality under the pressure of 0.65 MPa. The apparatus consisted of the test section of four straight sections and three U-bends, preheater, condenser, refrigerant pump, mass flow meter etc. The average heat transfer coefficient at the downstream straight section after U-bend was affected by U-bend due to the centrifugal force and mixing of two-phase flow in the U-bend. The average heat transfer coefficient at the U-bend was 4 ~ 33 % higher than that at the straight section. The average heat transfer coefficients were affected in the order of mass flux, heat flux and inlet quality. The average heat transfer coefficients in the microfin tube were lager by 19 ~ 49% and 33 ~ 69% than that in the smooth tube at the straight section and at the U-bend separately. The average heat transfer coefficients for R-407C were larger by 33 ~ 41% and 17 ~ 29% than that for R-22 in the smooth tube and the microfin tube separately.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.850-858
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• 2002

The present study investigates in detail the combined effects of the Coriolis force and centrifugal force on the development of turbulent flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a curved region of U-bend, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal force due to the curvature of U-bend. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct. On the other hand, under negative rotation, where these two forces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces. Under the condition that the value of Rossby number and curvature ratio is large, the flow field in a rotating U-bend can be represented by two dimensionless parameters : $K_{TC}$ =Re $\sfrac{1}{4}$√λand a body force ratio F=λ/Ro. Here, $K_{TC}$ has the same dynamical meaning as $K_{TC}$ =Re√λ for laminar flow.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.1
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• pp.159-169
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• 2002

The present study investigates in detail the combined effects of the Coriolis and centrifugal farce on the development of laminar flows in a square-sectioned U-bend rotating about an axis parallel to the center of bend curvature. When a viscous fluid flows through a rotating curved region, two types of secondary flow occur. One is caused by the Coriolis force due to the rotation of U-bend and the other by the centrifugal farce due to the curvature of U-bend. When the values of Rossby number and curvature ratio are large, the flow field in a rotating U-bend can be represented by two dimensionless parameters ; the Dean number K$\_$LC/=Re/√λ and a body ratio F=λ/Po. For positive rotation, where the rotation is in the same direction as that of the main flow, both the Coriolis force and the centrifugal force act radially outwards, the directions of the two secondary flows are the same. Therefore, the flow structure is qualitatively similar to that observed in a stationary curved duct with a larger f7c. On the other hand, in case of negative rotation, where two farces act in opposite direction, more complex flow fields can be observed depending on the relative magnitudes of the forces.

• 한국연소학회:학술대회논문집
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• 1998.10a
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• pp.91-101
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• 1998

The effect of secondary flow on both methane/air and propane/air premixed flame was investigated experimentally. By changing the radius of curvature, various flame behavior was observed. In the V-bend nozzles, flame surface is deformed from axisymmetry. As the exit velocity increased, flame lifted off partially. When the radius of curvature of the V-bend increased, the region where premixed flame is entirely on the rim increased. Since the axial velocity field is changed due to the secondary flow effect, comparison of V-bend and straight tube with the same diameter shows larger V-bend nozzle exit velocity for both flash back and flame blowout. The flame characteristics are mapped with a equivalence ratio, a velocity, and a nozzle radius of curvature. To identify physical reasoning on the flame surface deformation, numerical calculations are conducted. OH radical distributions in flames are visualized by PLIF technique.

• ETRI Journal
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• v.36 no.5
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• pp.841-846
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• 2014

This study proposes a novel optical sensor structure based on a refractometer combining a bend waveguide with an air trench. The optical sensor is a $1{\times}2$ splitter structure with a reference channel and a sensing channel. The reference channel has a straight waveguide. The sensing channel consists of a U-bend waveguide connecting four C-bends, and a trench structure to partially expose the core layer. The U-bend waveguide consists of one C-bend with the maximum optical loss and three C-bends with minimum losses. A trench provides a quantitative measurement environment and is aligned with the sidewall of the C-bend having the maximum loss. The intensity of the output power depends on the change in the refractive index of the measured material. The insertion loss of the proposed optical sensor changes from 3.7 dB to 59.1 dB when the refractive index changes from 1.3852 to 1.4452.

• International Journal of Air-Conditioning and Refrigeration
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• v.9 no.2
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• pp.85-93
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• 2001

The present study experimentally investigated the effect of refrigeration oil on the condensation heat transfer for R-22 and R-407C in a microfin tube with a U-bend. Mineral oil and POE oil were used for R-22 and R-407C respectively. Experimental parameters were an oil concentration from 0 to 5%, a mass flux from 100 to 400 $kg/m^2s$ and an inlet quality from 0.5 to 0.9. The enhancement factors for both R-22 and R-407C refrigerants at the first straight section decreased continuously as the oil concentration increased. The decreased rapidly as the mass flux decreased and the inlet quality increased. The heat transfer coefficients in the U-bend showed the maximum at the 90$^{\circ}$position. the heat transfer coefficients at the second straight section within the dimensionless length of 48 were larger by a maximum of 33% than the average heat transfer coefficient at the first straight section.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.6
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• pp.541-549
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• 2000

The present study experimentally investigated the effect of refrigeration oil on the condensation heat transfer for R-407C and R-22 in a microfin tube with a U-bend. POE oil ($74.1\;\textrm{mm}^2/s,\;40^{\circ}C$,) and mineral oil $62.5\;\textrm{mm}^2/s,\;40^{\circ}C$,) were used for R-407C and R-22 respectively Experimental parameters were an oil concentration from 0 to 5%, a mass flux from 100 to $400kg/m^{2}s sand an inlet quality from 0.5 to 0.9. The enhancement factors for both R-22 and R-407C refrigerants at the first straight section decreased continuously as the oil concentration increased. They decreased rapidly as the mass flux decreased and the inlet quality increased. The heat transfer coefficients in the U-bend showed the maximum at the $90^{\circ}$/TEX> position. The heat transfer coefficients at the second straight section within the dimensionless length of 48 were larger by a maximum of 33% than the average heat transfer coefficient at the first straight section.

• Journal of Convergence for Information Technology
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• v.12 no.3
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• pp.141-150
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• 2022

Fluid flow and thermal characteristics of laminar nanofluid(water/Al2O3) flow in a circular U-bend tube have been studied numerically. In this study, the effect of Reynolds number and the solid volume fraction and the impact of the U-bend on the flow field, the heat transfer and pressure drop was investigated. Comparisons with previously published experimental works on horizontal curved tubes show good agreements between the results. Heat transfer coefficient increases by increasing the solid volume fraction of nanoparticles as well as Reynolds number. Also, the presence of the secondary flow in the curve plays a key role in increasing the average heat transfer coefficient. However, the pressure drop curve increases significantly in the tubes with the increase in nanoparticles volume fraction.

• Proceedings of the KSME Conference
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• 2000.04b
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• pp.90-95
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• 2000

Condensation heat transfer experiments for R-22 and R-407C refrigerants mixed with mineral oil and POE oil respectively were performed in straight and U-bend sections of a microfin tube. Experimental parameters were an oil concentration from 0 to 5%, a mass flux from 100 to $400 kg/m^2s$ and an inlet quality from 0.5 to 0.9. The enhancement factors for R-22 and R-407C refrigerants at the first straight section decreased continuously as the oil concentration increased. They decreased rapidly as the mass flux decreased and inlet quality increased. The heat transfer coefficients in the U-bend were the maximum at the $90^{\circ}$ position. The heat transfer coefficients at the second straight section within the dimensionless length of 48 were larger by a maximum of 33% than the average heat transfer coefficients at the first straight section.